Co-rotating twin screw extruders are widely used not only for production, compounding and processing of plastics but also in other industries such as rubber, food, paint and pharmaceutical processing. Co-rotating extruders are built today in a modular manner with different processing elements mounted on screw shafts that allow the extruder to be adapted to different processing requirements. As opposed to single screw machines where the screw flight scrapes the inside of the housing (with clearance), an essential aspect of closely intermeshing co-rotating extruders is that the flights mesh tightly, except for the necessary clearance, and the screws are considered as “self-wiping” or “self-cleaning” with the flights designed to clean each other. The evolution, principles of operation and design principles of co-rotating twin screw extruders are well known and have also been simply explained in the book ‘Co-rotating Twin Screw Extruders—Fundamentals, Technology and Applications by Klemens Kohlgruber’ published by Carl Hanser Publishers of Munich (2008).
Besides intake and conveying of material, the most significant task carried out by a co-rotating extruder is mixing of the material to produce a homogeneous melt. Mixing in a co-rotating extruder is broadly of two types: distributive mixing and dispersive mixing. Referring to Chapter 9, pages 159-169 of, the book authored by Klemens Kohlgruber referenced above, distributive mixing refers to the distribution of different components or particles in the volume under consideration without necessarily reducing the size of the components or particles. In pure distributive mixing, high flow forces don't necessarily have to be applied. The type and number of re-arrangement processes, not the absolute magnitude of the shear and extension rates are decisive for mixing quality. With an infinite mixing time, i.e., infinite re-arrangements, a completely homogeneous mixture theoretically results. In contrast to distributive mixing, dispersive mixing initially involves disaggregating or dispersing the solid particles, liquid droplets or gas bubbles and generally involves reduction in particle or component size. For dispersion, sufficiently high flow forces must be applied in order to break up agglomerates or overcome surface tension at the interface between the melt and the liquid. Dispersive mixing depends on the absolute magnitude of the shear and extension rates as well as on the duration of the stress.
As also described in U.S. Pat. No. 6,974,243, for dispersive and distributive mixing usually kneading blocks which comprise a plurality of kneading disks with an Erdmenger profile, arranged axially one behind the other and offset angularly with respect to one another, are used. The kneading disks are respectively arranged in pairs, lying opposite one another on the two screw shafts of the co-rotating extruder, and closely intermesh. The mixing process in conventional kneading blocks is to be regarded as a random process, i.e. the mixing work performed in individual volume elements varies in intensity. Therefore, to achieve a high degree of homogeneity of the mixture, considerable mechanical energy has to be expended to ensure that, as far as possible, every volume unit also undergoes shearing. On the basis of an individual kneading disk, a relatively small proportion of the material to be handled is in each case sheared extremely intensely, while by far the greatest part of the material evades the shearing gap between the shearing disk and the barrel wall or between two kneading disks and between the two kneading disks and is consequently sheared only little. For this reason, to ensure a high degree of homogeneity of the mixture, either very long kneading blocks of the known type or else high rotational speeds are required. In any event, considerable mechanical energy is expended and is introduced in the form of heat into the material to be handled. In particular during the processing of rubber mixes, the generation of relatively large amounts of heat is extremely undesirable. U.S. Pat. No. 697,423 also describes elements that transition from a single lobe to a tri lobe and back. However, the element disclosed does not provide for “self-wiping”.
While both distributive and dispersive mixing are desirable for a more uniform melt, optimization of the extruder element is generally a compromise of the advantages and disadvantages of both types of mixing. U.S. Pat. No. 5,932,159 describes various types of extruder elements known for distributive and dispersive mixing. Increasing the distributive mixing ability in co-rotating extruder elements typically results in a loss or degradation of the wiping ability of the extruder.
It has therefore been a long felt need to have an extruder element for co-rotating extruders that eliminates or reduces the peak shear experienced by material, increases distributive mixing for more homogeneous mixing and better melt temperature control and also maintains the self-wiping ability of the extruder.